Some oil wells or other bore holes in the earth are drilled by rotating a drill pipe at the ground surface with a rock bit on the lower end of the pipe for drilling the earth. In such rotary drilling the entire drill string rotates. Other wells or parts of wells are drilled by lowering a hydraulic motor down the bore hole with a rock bit connected to the motor. Drilling fluid, commonly referred to as mud, is pumped down a pipe connected to the motor. The drilling fluid drives the motor which rotates the rock bit. The mud returns to the ground surface through the annulus surrounding the pipe in the bore hole. In this arrangement it is not necessary to rotate the entire drill string.
Either of two types of downhole hydraulic motors can be used. One known as a turbo drill has a large number of turbine stages driven by the flowing mud. Another type is a progressive cavity positive displacement motor analogous to a Moyno pump driven in reverse by the flowing drilling fluid. Either of such motors is a few inches in diameter and can range up to 30 feet in length. The housing of such a motor remains stationary and a shaft within the motor is rotated.
Between the motor and the bit for drilling there is a bearing assembly. Such an assembly has a fixed casing which is threaded to the casing for the motor and a shaft which is connected to the motor shaft. The bearing assembly includes radial journal bearings between the shaft and housing and thrust bearings for carrying the substantial thrust loads involved in this mode of drilling.
High pressure drilling fluid is applied through the pipe to the motor while the bit is off of the bottom of the hole. This generates a high thrust load tending to push the shaft downhole. This is referred to as an off bottom load. After the bit is rotating, the assembly is lowered so that the bit is in engagement with the bottom of the hole with sufficient pressure to effect drilling. This reverses the direction of thrust in the bearing assembly and is referred to as the on-bottom thrust. Several such thrust reversals can be encountered as drilling is stopped and started each time a length of drill pipe is added to the string or for other reasons. The thrust bearings are subjected to high loads, vibration, and in some cases rather high rotational speeds. Exemplary speeds in a positive displacement motor are in the range of 125 to 500 RPM. Turbo drill speeds can be four times larger and are typically 1000 RPM. The bearing assembly must last hundreds of hours so as to outlast the rock bit.
The off-bottom load to be carried by the thrust bearings can be as much as 30,000 pounds when a positive displacement motor is used. Off-bottom thrust can be as much as 40,000 pounds with rotational speeds as high as 2000 RPM with a turbine motor, although such conditions are preferably avoided. The on-bottom thrust loads at low to intermediate speeds (e. g., 125 to 400 RPM) range from 10,000 to 40,000 pounds. At high speeds (e.g., 1000 RPM) thrust loads can range up to 20,000 pounds or more.
Thrust bearings using balls operating in thrust carrying races have been used for the thrust bearings between a downhole drill motor and a rock bit. A substantial problem with ball bearing type thrust bearings is the ability to sustain high drilling loads for long enough periods of time. The ball bearing thrust bearings presently available are not suitable for carrying thrust of more than 20,000 pounds on a sustained basis, particularly at high speeds. When the assembly must carry very high thrust loads in either of two directions, a rather large number of separate ball bearing stages must be used so that no individual stage is excessively loaded. This results in a bearing assembly that is extraordinarily long. The assembly may need to be sealed to retain lubricant for the bearings and seals are difficult in the downhole conditions. Such assemblies are also costly to manufacture, assemble and adjust to the required precision.
Friction bearings of rubber and/or plastic have been made for turbo drills where one attempts to balance the downhole thrust from the motor with the uphole thrust of the drill bit to minimize thrust load on the bearing assembly. Such balance is difficult to achieve in practice and many stages of thrust bearing can be required to carry the loads under all conditions. Further, such bearing assemblies using organic materials have maximum temperature limitations that restrict their use and they may not be usable with oil base drilling muds. Metal inserts in rubber have also been tried but these are subject to the limitations of the organic materials and further, it is difficult to form a good enough bond between the metal and rubber to withstand temperature cycling or even somewhat elevated temperatures.
It is desirable to provide a high load thrust bearing assembly that is capable of carrying heavier loads for sustained periods than the thrust bearing assemblies presently available. Bearing assemblies operated under the severe conditions of well drilling have operating lifetimes of up to only about 250 hours. It is desirable to provide a thrust bearing system capable of operating over considerably longer time periods. It is also desirable to provide a thrust bearing system that does not require precise adjustment and can accept shock loading and the like without damage to the bearing assembly or the drill motor.
Such a bearing assembly should be usable with oil or water base drilling muds and operate under any temperature conditions that a rock bit can tolerate. The assembly should tolerate the wear encountered during its useful life. It should be operated without need for rotating seals and capable of withstanding numerous thrust reversals. Preferably the assembly is self-aligning and readily built without need for precise adjustments.